1. Field of the Invention
The present invention relates to a DC-DC converter and a light emitting diode (LED) driving circuit using the same. More particularly, the invention relates to a Pulse Width Modulation (hereinafter referred to as PWM) based DC-DC converter, which immediately blocks over-current or over-voltage due to short-circuit or disconnection from being applied to the DC-DC converter in order to protect a circuit, and an LED driving circuit using the same.
2. Description of the Related Art
In general, a Cold Cathode Fluorescent Lamp (CCFL, hereinafter referred to as CCFL) used for a light source of a Liquid Crystal Display (LCD, hereinafter referred to as LCD) uses mercury gas. Thus it has drawbacks in that it is prone to environmental pollution, has a slow response rate, has low color reproducibility and is not appropriate for miniaturization of an LCD panel.
On the other hand, a Light Emitting Diode (LED, hereinafter referred to as LED) has merits in that it is environmentally friendly, has a prompt response rate in nanoseconds, which is effective for a video signal stream, is possible in impulsive driving and has color reproducibility of over 100%. Further, not only the amount of light of red, green and blue LEDs can be adjusted to change luminance and color temperature, but also the LED is appropriate for miniaturization of an LCD panel. Thus, recently, the LED has been actively adopted as a light source for a backlight such as an LCD panel.
In case of using a plurality of LEDs connected in series in a LCD backlight, a driving circuit is needed to provide a controlled level of constant current to the LED. A PWM based DC-DC converter is typically used for a circuit to drive an LED with constant current. FIGS. 1(a) and (b) illustrate a conventional PWM based DC-DC converter.
First, FIG. 1(a) is a circuit diagram illustrating an example of a conventional LED driving circuit using a buck DC-DC converter. As shown in FIG. 1(a), in the conventional buck DC-DC converter, an inductor L is connected in series with an LED array 11 at a positive end of a direct current Vin, and a diode D is connected in parallel with the inductor L and the LED array. In addition, a switch 12 and a voltage detection resistor Rs is connected in series at a negative end of a connection node of the diode D and a negative end of the direct current Vin. The voltage value detected by the voltage detection resistor Rs is inputted to a PWM controller 13, which adjusts an on/off duty ratio of the switch according to the detected voltage value. The switch 12 can be a metal oxide semiconductor field-effect transistor (MOSFET) as shown in FIG. 1, which can be used as a switch by adjusting a gate voltage thereof.
When the switch 12 is ‘on’, the current supplied from the direct current Vin is transmitted to the LED array 11 via the inductor L. At this point, energy is accumulated in the inductor L. When the switch 12 is ‘off’, power is supplied to the LED array by the energy accumulated in the inductor L. The PWM controller 13 adjusts an on/off duty ratio of the switch 12 according to a resistance value of the voltage detection resistor Rs.
In an LED driving circuit using such a buck DC-DC converter, when both ends of the LED array 11 are short-circuited due to abnormality in the external environment of the circuit or in the driving circuit itself, over-current may run on the circuit, damaging the diode D and the switch 12. The LED included in the LED array 11 can also be damaged by the over-current if it exceeds the rated current of the LED array 11. Furthermore, the accumulated current may flow from Switch Mode Power Supply (SMPS), which supplies the direct current Vin, to the driving circuit, causing a more serious problem.
FIG. 1(b) is a circuit diagram illustrating an example of an LED driving circuit adopting a conventional boost DC-DC converter. As shown in FIG. 1(b), in the conventional boost DC-DC converter, an inductor L and a diode D are connected in series at a positive end of a direct current Vin, and a capacitor C and an LED array 11 are connected in parallel between the diode D and the direct current Vin. A switch 12 and a voltage detection resistor Rs are connected in series between a connection node of the inductor L and the diode D and a negative end of the direct current Vih. A voltage value detected by the voltage detection resistor Rs is inputted into a PWM controller 13, which adjusts an on/off duty ratio of the switch according to the detected voltage value. As shown in FIG. 1(b), the switch 12 can be a MOSFET, which can be used as a switch by adjusting a gate voltage thereof.
When the switch is ‘on’, the current supplied from the direct current Vin runs through the inductor L and the switch S, and energy is stored in the inductor L. When the switch 12 is ‘off’, the sum of the energy accumulated in the direct current Vin and that in the inductor L is passed through the diode D and transmitted to the LED array. Here, the voltage is smoothed by a smoothing capacitor C before being transmitted to the LED array 11, and the value of the voltage is equal to or greater than an input voltage Vin.
In an LED driving circuit using such a boost DC-DC converter, if at least one of the LEDs of the LED array 11 is disconnected, the current does not run on the LED array 11. Thus, due to counter electromotive force of the inductor, both ends of the LED array 11 are applied with a voltage several times higher than the inductor voltage during a normal operation and the input voltage, resulting damage to the device by over-voltage.
Therefore, there has been a need for a DC-DC converter circuit having an over-voltage or over-current protection function, preventing damage to the device due to over-current or over-voltage generated when an LED (a load) is shorted or disconnected.